～とする
try to / about to, suppose that, as a role
～とする presents something not simply as a fact, but as the basis or frame the rest of the sentence works from. After a volitional form (～よう / ～おう), it describes an action that is about to happen or is being attempted: “try to ~, be about to ~”. After a plain clause, it introduces a hypothesis ("suppose that..."). After nouns, it formally treats or designates one thing as another ("regard A as B", "have A as B").
1. Attempt or the verge of an action (volitional + とする)
Volitional + とする describes an action that was attempted or just about to happen, often one that gets interrupted or does not succeed.
家を出ようとしたとき、電話が鳴った。
“Just as I was about to leave the house, the phone rang.”
- 家（いえ）= house, home
- を = object marker
- 出ようとした（でようとした）= was about to leave (volitional 出よう plus とする)
- とき = when, the moment
- 電話（でんわ）= telephone
- が = subject marker
- 鳴った（なった）= rang
2. Assumption: “suppose that ~”
After a plain clause, often as とすると / としたら / とすれば, it lays down a hypothesis and reasons from it.
明日雨が降るとすると、試合は中止になるだろう。
“Supposing it rains tomorrow, the match will probably be called off.”
- 明日（あした）= tomorrow
- 雨（あめ）= rain
- が = subject marker
- 降る（ふる）= to fall (of rain)
- とすると = supposing that
- 試合（しあい）= match, game
- 中止になる（ちゅうしになる）= to be called off
- だろう = probably
3. Treating A as B
AをBとする formally treats or designates A as B. This use is common in written and official Japanese.
私たちは彼をリーダーとするチームを作った。
“We formed a team with him as its leader.”
- 私たち（わたしたち）= we
- 彼（かれ）= he, him
- を = object marker
- リーダーとする = having as leader, designating as leader
- チーム = team
- 作った（つくった）= made, formed
How to form it
Verb volitional + とする (attempt), plain clause + とすると／としたら (assumption), or Noun を Noun とする (role).
- 出る（でる）= to leave → 出ようとする（でようとする）= to be about to leave
- 行く（いく）= to go → 行こうとする（いこうとする）= to try to go
- 降る（ふる）= to rain → 降るとすると（ふるとすると）= supposing it rains
Key idea
とする sets up a framework the rest of the sentence works within. That framework may be an action that is about to happen, a hypothetical situation, or a formal way of treating or designating something.
Other N2 grammar
Updated .